The present invention concerns an annular fluid-tight packing for male-female cast iron pipe joints. The packing is made of a single hardness elastomer, is radially compressed during the insertion of the male end, and accommodates wide dimensional variations in the internal diameter of the female housing and the external diameter of the male end.
It is known that with this type of packing the relative dimensions of different constituent parts of the joint, that is, of the fluid-tight packing of the male end and of the housing are functions of:
the dimensional tolerances of the pipes;
the minimum compression of the packing required to ensure fluid-tightness under the most unfavorable conditions, such as a maximum internal diameter of the housing, a minimum external diameter of the male end, a non-axial housing interior; and irregular pipe surfaces; and
the maximum admissible compression for the packing beyond which premature wearing of the elastomer occurs and the stress from coupling the male end into the housing becomes excessive.
In addition, the ease and reliability of the joint assembly and specifically the prevention of the pulling of the packing out of its place in the housing when the male end is inserted into the housing are strictly determined by the respective shapes of the radial section of the packing and the internal contour of the housing.
Furthermore, in the case of the assembly of a joint under the most unfavorable conditions described above, the annular space between the internal surface of the housing and the external surface of the male end can form a clearance such that a localized expulsion of the packing towards the exterior becomes possible under the effect of internal hydraulic pressure.
On the other hand, the body of the packing, in radial detachment with respect to its anchoring, is limited in its axial strain to its end in contact with the liquid in the pipeline by an internal shoulder of the housing.
Such an internal shoulder is necessary to prevent the pulling of the packing into the pipeline upon the insertion of the male end, which the anchoring is insufficient to prevent. Thus, when the packing body compresses to ensure fluid-tightness it cannot extend into the annular space between the housing and the male end, which considerably increases the force required for the introduction of the male end for a slight increase in its external diameter.
Such problems demonstrate the difficulty in achieving an elastic and yet fluid-tight packing for joining cast iron pipes which satisfies conditions as antagonistic as fluid-tightness at a surface pressure of at least 40 bars with extreme off-centering between the male end and the housing, a slipping stress which is not excessive and dimensional tolerances over the diameter of the pipes, and the longevity of the packing itself in chemically aggressive surroundings.
In U.S. Pat. No. 3,315,971 a fluid-tight radial compression packing is known for cast iron pipes in which a compressed flange is extended on the side of the housing bottom by a flexible annular lip stretched circumferentially and sealingly applied on the external male end cylindrical wall by the pressure of the liquid in the pipeline. However, with a low pressure the liquid can penetrate between this flexible lip and the external wall of the male end until it reaches the compressed flange, the fluid-tight housing thus being limited to a simple radial compression flange totally fixed in the housing and the flexible lip being ineffective. This leakage past the flexible lip is all the more probable as the dimensional tolerances on the diameter of the male end increase. In addition, the fluid-tight flange is exposed to oxygen if the joint is in the air or solid earth particles if the joint is buried. The result is chemical and physical aggravation to the flange, which shortens the length of service of the packing and causes leaks in the joint.
Finally, the tension of the lip makes it sensitive to the vapors of natural gas condensation, and prevents it from ensuring sound fluid-tightness with the external surface of the male end when the surface bears depressions and irregularities, the circumferentially extended lip being unable to fill the depressions.
German utility model No. GM 19 29 826 teaches an entry and guide ring for the male end within the housing, such that an anchoring for pipes of polyvinyl chloride or PVC in which a projection located radially opposite the anchoring and directed towards the bottom of the housing and having a triangular radial section, maintains the rectangular anchoring in a groove on the internal surface of the housing. However, in pipelines made of plastics one must prevent the ring from exerting too great a pressure on the external surface of the male end, to the point where it creates a localized constriction. This restriction thus limits the fluid-tightness ensured by the triangular foot or projection, as well as the possibility of maintaining the anchoring in its groove.
Furthermore, the anchoring is so substantial that its axial dimension is only slightly less than the total axial dimension of the ring, and constitutes with the opposite foot the largest part of the ring body. The result is that, if one wishes to equip such a ring with a radial compression body of sufficient size, the ring volume creates an unacceptable axial encumberment for the housing.
In addition, the diametral tolerances possible in plastic pipelines are tens of millimeters, whereas for the cast iron pipelines they are counted in millimeters. Furthermore, polyvinyl chloride pipelines have a maximum diameter of 400 mm. Finally, in this type of pipeline the housing is as flexible as the male end, whereas with cast iron pipelines the housing is much more rigid than the male end.
In French Pat. No. 2,031,925 a fluid-tight packing is disclosed for pipes with a male end and a housing, characterized by an anchoring presenting a first lobe projecting on the internal surface of the packing to ensure, through a slight compression, the stability of the anchoring in its groove on the internal surface of the housing. A second more significant lobe located at the end of the ring on the bottom side of the housing ensures, through its compression, the fluid-tightness of the joint. But in this type of packing the first lobe is not placed opposite the anchoring and the projection is quite weak, such that the anchoring is not always lodged in its groove. The bottom of the housing also has a shoulder which limits the packing extension in the axial direction upon compression and all the more increases the stress required in the assembly of the joint, especially when the male end has a maximum external diameter and the housing has the minimum internal diameter.